On Tuesday, 90 percent of the chain's 6,000 U.S. stores began accepting mobile orders and payments through an iPhone and Android app. And just 24 hours after launch, Taco Bell said 75 percent of all stores already had processed a mobile order.

Taco Bell surprised customers when it wiped out its social media channels earlier this week to promote the app. Its Twitter, Facebook and Tumblr pages are still "blacked out" with a single post promoting the hashtag #onlyintheapp, effectively driving followers to the app.

"We needed to do something right out of the gate that people would notice, and what better way to notice and drive people to the new way to Taco Bell than the blackout?" said Chris Brandt, Taco Bell's CMO. "We will continue to be active in social media, we will continue to be disruptive, we're not walking away from all of those channels where we've built up a great discourse with our fans."

Now, Taco Bell has started amplifying its blackout post with Promoted Tweets and other social ads. The fast-food chain plans to limit all other social media content for the next couple days.

"In the past when we launched products, we focused on the product, we focused on why it's so good, and we connected that to lifestyle—using social to bring those together," said Dave Marsey, managing director of DigitasLBi San Francisco, Taco Bell's digital agency behind the campaign. "In this case, what led us to #onlyintheapp is that it's a new way—these are new ways to experience Taco Bell."

Fast Food Goes Mobile

Taco Bell declined to disclose how many people have downloaded the app but said that it was No. 1 in the food and beverage section of Apple's App Store on Tuesday.

The chain has some tough competition in mobile, though. Starbucks will reportedly test an order-ahead feature at 150 Portland locations later this year, and Burger King, McDonald's and Wendy's have also launched mobile payments within the past year.

But unlike many of the other apps from quick-service restaurants, Taco Bell's app does not require employees to scan a bar code or type in a code. Instead, customers link their orders to a credit card and then pick the food up at designated places in stores.

Per Brandt, Taco Bell accepts more drive-through orders than in-store orders, which is why the app was developed for both. So far, app orders are split evenly between the two types, but he expects to eventually start accepting more for the drive-through. To get employees up to speed on how the app works, Taco Bell doubled its usual training time for a product launch.

The restaurant's app also doles out exclusive offers. Right now, the chain is pushing its Starburst Freeze drinks for $1. Going forward, Taco Bell hopes to personalize the deals with data. For example, offers can be day-parted based on when someone visits the chain the most.

"The treasure trove of information that we'll get about the consumer and their behaviors [and] habits will be awesome," Brandt said. "We think we can make it a richer experience for us and the consumer by giving them offers tailored to their behavior."